1. Field of the Invention
The present invention relates to a cutting method using a vibratory cutting tool which feeds a cutting edge while vibrating in its feed direction relative to a workpiece rotated around an axis so as to cut the workpiece by turning.
2. Discussion of the Background
In turning an outer periphery of a workpiece using a common cutting tool, a cutting tool main body 1 is fed at a constant speed in a feed direction F parallel to an axis 0 while rotating a workpiece W around the axis 0 in a rotation direction C, as shown in FIG. 9, and the outer peripheral surface of the workpiece W is cut by a cutting edge 2 attached to the tip of the cutting tool main body 1. The amount of movement of the cutting edge 2 in the feed direction F during one rotation of the workpiece W is a feed f, and the locus L.sub.o of the cutting edge 2 on the outer peripheral surface of the workpiece W becomes a spiral twisted at a fixed angle.
Therefore, the feed f can also be regarded as the spacing between the spirals in the feed direction F drawn by the locus L.sub.o of the cutting edge 2 on the outer peripheral surface of the workpiece W.
However, according to the turning using such a common cutting tool, as the locus L.sub.o of the cutting edge 2 becomes a spiral of a fixed angle as described above, continuous, flow-type chips are produced and discharged by the cutting edge 2. Such continuous chips are undesirable because they wind themselves around the workpiece W and the cutting tool main body 1 or they get caught in a chuck of a lathe so as to be rotated at high speed, so that they might interfere with a smooth cutting operation or harm a machined surface of the workpiece W and the cutting edge 2.
Thus, the separation or breaking of the chips produced by the cutting edge 2 solves the above-mentioned problems. A cutting method by a vibratory cutting tool has been proposed in which a cutting edge 2 of a cutting tool is fed in the feed direction F while being vibrated at high speed in a direction of the axis 0 of a workpiece W so as to cut the workpiece W.
FIGS. 10 and 11 show an example of such a cutting method by a vibratory cutting tool. The cutting edge 2 is fed together with the cutting tool main body 1 in the feed direction F while being vibrated in the direction of the axis 0 of the workpiece W at a fixed period.
Incidentally, as a means for vibrating the cutting tool main body 1 in such a vibratory cutting tool, for example, a means for swingably supporting the cutting tool main body 1 at a center portion thereof and bringing a cam rotated by a motor, etc. into abutment with the rear end of the cutting tool main body 1 in the direction of axis 0, and a vibratory cutting tool such as described in Japanese Laid-Open Publication HEI 8-300207 (Japanese Patent Application No. 7-108670) which was previously filed by the present Applicant in which the cutting tool main body 1 is provided with an elastically deformable low-stiffness section, with a portion ahead of the low-stiffness section intermittently pressed in the direction of axis 0 by a direct-acting actuator so as to vibrate the cutting edge 2 at the tip of the cutting tool main body 1 can be employed.
In the example shown in FIG. 10, the cutting tool main body 1 is subjected to a sine waveform vibration as shown in FIG. 11. The locus L.sub.o is formed under conditions of no vibration. The locus L of the cutting edge 2 extends spirally about the locus L.sub.o, while moving wavily in the direction of axis 0.
However, if the amplitude a of the vibration is less than f/2, as shown in FIG. 11, and the relationship represented by the following expression (1) are satisfied when letting a period of rotation of the workpiece W be T and a period of vibration of the cutting tool main body 1, i.e., the cutting edge 2, be t.sub.total, a position A where the cutting edge 2 moves farthest in the feed direction F (hereinafter, referred to as a front end position), and a position B where the cutting edge 2 moves farthest opposite to the feed direction F (hereinafter, referred to as a rear end position) are located close to each other in the direction of axis 0 in the region between both spiral loci L of the cutting edge 2 in the direction of axis 0: EQU 2T=t.sub.total .times.N (1)
where N is an odd number of 1 or more.
That is, with respect to a certain front end position A on the locus L, the rear end position B is located at a position the cutting edge reached after going round in the direction opposite to the rotation direction C of the workpiece W along the locus L from the front end position A, and hence, the spacing in the direction of axis 0 between the spirals drawn by the locus L of the cutting edge 2 become narrow at a portion where the front end position A and the rear end position B are close to each other. Furthermore, the width of the chips produced by the cutting edge 2 also become narrow at this portion, thus they are likely to be broken. Incidentally, FIGS. 10 and 11 show the example of a case where N=5.
In addition, FIG. 12 shows a case where the amplitude a of the vibration of the cutting edge 2 equals f/2, and N=5. In this case, the front end position A coincides with the rear end position B, so the chips are completely separated.
Incidentally, when the above-described expression (1) is satisfied the locus L of the cutting edge 2 provides for a front end position A located at a position the cutting edge 2 reached after going round in the direction opposite to the rotation direction C of the workpiece W along the locus L from a certain rear end position B, and the spacing between the spirals drawn by the locus L in the feed direction F is a maximum at this portion. Here, as mentioned above, the feed f of the cutting edge 2 is the spacing between the loci L in the feed direction F. Thus, when letting the maximum value of the feed f be the maximum feed f.sub.max, the maximum feed f.sub.max in the above case corresponds to a distance in the feed direction F between the rear end position B and the front end position A in which the spacing between the spirals drawn by the locus L is a maximum. When a&lt;f/2, f.sub.max =f+2a, and when a=f/2 so that the chips are completely separated, f.sub.max =2f.
On the other hand, however, it is generally known that surface roughness of the workpiece W in the turning operation by the cutting tool deteriorates as the feed f increases. For example, the maximum height R.sub.max of surface roughness of the workpiece W in the direction of axis 0 (lateral roughness) is represented by the following expression (2) as an approximate expression: EQU R.sub.max =f.sup.2 /8R (2)
where R is the radius of the cutting edge 2.
That is, the surface roughness R.sub.max of the workpiece W is proportional to the square of the feed f. Therefore, when f.sub.max =2f so that the chips are completely separated by the vibratory cutting tool as mentioned above, for example, the surface roughness R.sub.max increases by four times the feed amount f causing no vibration of the cutting tool at a portion where the spacing between the loci L in the feed direction F is a maximum, thus causing remarkable deterioration in machined surface roughness.
In addition, at a portion on the locus L where the cutting edge 2 moves from the rear end position B to the front end position A, the feed f of the cutting edge 2 gradually increases to the maximum feed f.sub.max.
However, as the feed f of the cutting edge 2 locally increases, cutting resistance acting on the cutting edge 2 of the portion also increases. Thus, when the maximum feed f.sub.max is large, there arises a problem in that the cutting edge 2 is likely to be chipped due to increased cutting resistance.
The present invention is made under the circumstances as described above, and has its object to provide a cutting method using a vibratory cutting tool which can retard both a deterioration of machined surface roughness of a workpiece and an increase in cutting resistance while enabling efficient operation by using a vibratory cutting tool for breaking or separating chips.